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Topic: 5D3 + 50 F1.2 L (Read 8741 times)

The 50L is a pretty special lens. You can run into issues with focus shift at close distances when you shoot stopped down. I'd put it on a tripod at 8-10 ft, and try shooting a brick wall at f/1.2 (Camera carefully aligned) using the different focus points. This eliminates problens like focus shift, so you see the AF performance. Do not stop the lens down, check it at 1.2!

Take 10 shots at each point resetting the lens to infinity each time. If the outside points are oof, I'd return it either for exchange, send camera and lens to Canon or try a different lens.

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I use mine with a 5dII - which supposedly has "horrible" AF compared to it successor. And yes, "front focus" is part of the lens design. As far as I know that hasn't really bothered people too much for the last 80 or 100 years or however long this optic formula has been used now.AF is never 100% since it is focusing on something that has enough contrast that falls in the area of the focus point - which I believe are bigger than the little dots you see in the viewfinder. So results vary since the camera can't read our mind quite yet. And slight movements will have an impact also.What I'm saying is: if you're already obsessing about "sharpness" of the lens you might consider just returning it for something different that you like better. Maybe something is really out of whack with either the lens or the body. Maybe try another one and also another body and see if there's a noticeable difference under real life circumstances.If you can get over it shoot it wide open and enjoy the colors and contrast which is probably second best to the Leica/50mm combo.

I've noticed after using the 50L for awhile now that it has a field curvature outside the center point. My solution is multiple shots while slightly moving forward or if you focus and recompose it seems that it lands pretty close as well.

Then sometimes, the AF just nails it. The 50L is a fickle and unique beast, it just takes practice to learn it quirks.

I've noticed after using the 50L for awhile now that it has a field curvature outside the center point.

Exactly...that is the crux of 50L issues and strengths.... That "dreamy" look is the product of that curved focal plane...so using a centerpoint AF and recomposing is a fool's errand with this lens. Superimposed on this is the real focus shift issue which canon can address but again only to a point if they want to keep the optical formula simple, weight and price down.

85L II, a close cousin, also released the same year as the 50L, side steps this if only marginally...Being of longer focal length 85mm already has the advantage of some image compression at the focal plane...residual curvature can be "corrected" by additional glass designed to provide a more flat field, and I would hazard a guess this is the case for 85L II.

Could explain the massive size and weight we see with 85L II...not to mention the price.

You get the 50L because you know its strengths and draw backs ... in other words, it is what it is.

Folks, the 50L focal plane isn't curved or anymore curved than the average lens.

Imagine taking a single picture of a flat poster on a wall (for exaggeration, this could be f/1.2 at close range), do you suppose the picture is out of focus away from the center? The answer is no. That was field curvature, or the lack of, in a nutshell.

The subject of focus variance using different focus points (wrt other points) has everything to do with the excessive spherical aberration (SA) inherent to the 50L's optics. While this designed SA is great for its artistry, it is problematic for any phase-detect type auto-focusing systems. This same under-corrected SA is also responsible for the infamous focus shift.

Folks, the 50L focal plane isn't curved or anymore curved than the average lens.

Imagine taking a single picture of a flat poster on a wall (for exaggeration, this could be f/1.2 at close range), do you suppose the picture is out of focus away from the center? The answer is no. That was field curvature, or the lack of, in a nutshell.

I disagree. True, SA, CA, and coma all constitute aberrations, yes...but what the lay-person refers to as "curved focal plane" while not technically or semantically precise, is nonetheless not an imagined phenomenon. Petzval field curvature does exist.

Taking SA as an example, imagine if the points in sharp focus can be joined in a 3D graph across the "focal 3D space" and your sensor can be magically bent to precisely pass through these points of maximal focus... It will not be a flat plane...while one could hope for a nice smooth bowl like graph, some complex optical systems, at least in theory, could generate nice central plane with concentric ripple with circular throughs and waves around this plane.

The challenge for optics designers is to address/correct SA, CA, coma and other spurious aberrations and distortions that arise in multi-element optics in a fair compromise and yet keep price and weight down...I do however agree that some SA was intentionally incorporated into the f/1.2 systems for artistic effect.

But field curvature does exist and is in fact the norm, to varying extent in both corrected and uncorrected optical systems.

So when the fanboys want corner-to-corner sharpness in the f/1.2 systems with zero CA and that melting bokeh, one has to just smile and be nice

Folks, the 50L focal plane isn't curved or anymore curved than the average lens.

Apologize. I shouldn't have said focal plane. I'm not a techie. I guess I was referring to the field curvature. I just know that if you lock focus with the center point and recompose you will more than likely end up with an image that is at least slightly out of focus.

I disagree. True, SA, CA, and coma all constitute aberrations, yes...but what the lay-person refers to as "curved focal plane" while not technically or semantically precise, is nonetheless not an imagined phenomenon. Petzval field curvature does exist.

Taking SA as an example, imagine if the points in sharp focus can be joined in a 3D graph across the "focal 3D space" and your sensor can be magically bent to precisely pass through these points of maximal focus... It will not be a flat plane...while one could hope for a nice smooth bowl like graph, some complex optical systems, at least in theory, could generate nice central plane with concentric ripple with circular throughs and waves around this plane.

The challenge for optics designers is to address/correct SA, CA, coma and other spurious aberrations and distortions that arise in multi-element optics in a fair compromise and yet keep price and weight down...I do however agree that some SA was intentionally incorporated into the f/1.2 systems for artistic effect.

But field curvature does exist and is in fact the norm, to varying extent in both corrected and uncorrected optical systems.

So when the fanboys want corner-to-corner sharpness in the f/1.2 systems with zero CA and that melting bokeh, one has to just smile and be nice

Of course there will always be some however measurable curvature to the focal plane in the real world. Is the Earth truly round? Is a flat wall ever truly flat?

It was that very reason I took the time to qualified with "the 50L focal plane isn't curved or anymore curved than the average lens." For the intent and purpose, the focal plane of the 50L is not abnormally nor excessively curved. In layman's term, it’s flat or of no real concern.

Spherical Aberration is not remotely the same as Chromatic Aberration nor does it have anything to do with focal plane curvature. It does however have everything to do with the shape/curvature of the spherical lens and can be corrected by way of aspherical contouring and/or counter elements (e.g. floating element).

The phenomenon of front focusing with the peripheral AF points is an entirely different matter and surely isn't called Focal Plane Curvature. Instead what was observed is none other than misjudgment by the phase-detect AF system in the presence of excessive Spherical Aberration. Despite such flaw, each peripheral AF points are still able to arrived at a given focus plane (even if not actually in-focus), but more importantly, be uniformly out-of-focus (aka flat).

On the other hand, given a lens that does exhibit significant Focal Plane Curvature, one would be able to achieve perfect focus at any AF points (including peripherals), but the image would not be uniformly in-focus (only area near the center of focus). This phenomenon is independent of the type of focusing system used btw; AF or Manual. No amount of eye-balling MF will ever get you a flat pic. Clearly not what's going on here. We know the 50L is fully capable, especially MF'd or even Live-View contrast AF’d.

So what is it about SA that sends phase-detect AF to the funny farm?

Imagine at any given point on the actual in-focus plane, there exists a series of also in-focus points in-front and/or behind it (projected from different parts of the lens. see illustration). A typical phase-detection AF system simply doesn't have the smarts to make that judgment call. In a way, it did what it was designed to do, just didn't lock-on to the best one. This same mechanism is also the reason for Focus Shift at all AF points (even the center).

By stopping down (pinching of the aperture), you can cut down on the number of these false positives projections; less interference. For the 50L, the cut-off is somewhere around f/5.6-8.0 (DOF is large enough by then, that it’s tough to say for sure).

The bottom of the line is Canon had made a conscious decision to leave excessive (not little, but a lot) amount of SA uncorrected. Call it for the artistry or what have you, it’s there and it’s real – a real pain in the arse … until one learns to cope with it.

Through the ownership of the 50L, I have come to leverage the peripheral AF error against the center point focus shift.

AFMA wrt the center point is assumed. Focus shift at the center is some degree of back-focus when in the red-zone (within a several feet of subject, and f/1.2 > aperture > f/5.6). Knowing that switching from center to the peripheral points would result in some degree of front-focus, it could be used to negate the back-focus (from the focus shift). Examples:

At wide open (f/1.2) I know I can shoot fairly reliable without thinking much.

Inside the red-zone, I select one of the peripheral AF points best for the given aperture (you’ll have do some test runs for yourself to determine which pairs well).

Outside of the red-zone, I shoot normally (any F-stop with any AF points).

Folks, the 50L focal plane isn't curved or anymore curved than the average lens.

Apologize. I shouldn't have said focal plane. I'm not a techie. I guess I was referring to the field curvature. I just know that if you lock focus with the center point and recompose you will more than likely end up with an image that is at least slightly out of focus.

No apology. Field Curvature and Focal Plane Curvature are of the same btw. See my response above.

Shoot at f1.2, open the file in DPP, hit command J (I don't what it is for windows), look at the size of the focus point relative to the subject it covered. Most of the time (for my images) the DOF is thiner than the AF point!